Dry transformers are commonly used in medium voltage power systems and vacuum type circuit breakers are often utilized in connection with dry transformers. Vacuum circuit breakers could, during certain conditions, cause over voltage conditions when used for operating the transformer. These over voltages could cause transformer insulation failure when transient over voltage protection is not installed. Common transient over voltage protection includes surge arresters and oil-filled surge capacitors and damped surge capacitors (RC-snubbers).
One problem with the use of vacuum circuit breakers is that induced transient over voltages can be induced. This is especially the case when vacuum circuit breakers are opened with inductive loads attached to the system at phase angles causing multiple re-ignitions. Opening during the transformer inrush current or during a short circuit on the secondary side of the transformer are other examples of “inductive loads” not commonly thought of. Even though surge arresters may be utilized to limit transient voltage amplitudes at the point of installation, high stresses could be reached inside the transformer winding due to internal resonances.
Additionally, dry type transformers are now being used so as to avoid the problems associated with oil-filled transformers typically used in conventional medium voltage systems. Oil filled transformers present unique problems that dry transformers do not, for example, possible leakage of the oil creating environmental issues, and the possibility that the oil might catch fire such that, explosion proof transformers are often preferred today. Accordingly, when utilizing dry transformers it is of course highly undesirable to utilize oil filled surge capacitors, which present the same or similar problems discussed in connection with oil filled transformers although the oil quantity is much less for the capacitor compared to the transformer.
U.S. Pat. No. 4,604,673 (the '673 patent) discloses a distribution transformer that includes a surge protection device for protecting against external lightning strikes. For example, the '673 patent describes an oil filled transformer equipped with a varistor stack mounted within the oil tank and connected between a midpoint of the winding, and two ends of the winding. However, the '673 patent fails to disclose or teach a protection system that is capable of protecting against internal resonances developed within a dry type three phase transformer. Rather, the '673 patent is directed toward protecting oil filled single phase overhead (pole-type) distribution against external lightning strikes. Col. 1, II. 16-19 and 62-66. Likewise, the single phase system disclosed in the '673 patent could not be adapted in a three phase delta connected transformer. However, this is not an issue for the system taught in the '673 patent as it is a single phase system utilizing an oil filled transformer.